This invention relates to liquid crystal devices and, more particularly, to liquid crystal devices useful for dot matrix displays.
Liquid crystal devices are useful for display purposes by their property of passing light when electrically energized and being opaque to light when not energized. They are particularly useful in that very little power is needed to energize them since they merely change the state of passing light which is externally generated or exists in the ambient.
Another device in the display art is a light-emitting diode (LED). The LED which is electrically equivalent to a diode can be arranged in a dot matrix very easily by arranging the LEDs in rows and columns that are energized in a conventional cross-point diode matrix manner. Accordingly, the LED is easy to program to provide a desired dot matrix display. However, the LED requires much more power and voltage for the electrical control than a liquid crystal device (LCD) arranged in a dot matrix, since in the LED the light is generated by the LED itself. Moreover, it is much more difficult to control a dot-matrix liquid crystal display than a light emitting diode dot-matrix display. The reason for this difficulty is primarily due to the lack of the so-called diode isolation effect that is manifested in the LED-type matrix but which cannot be produced between the elements in a conventional liquid crystal display matrix. For example, when a row and column of an LCD matrix are selected, both the entire row and entire column are energized. The conventional way of controlling LCD displays, that is, by assigning a pin to each LCD dot is impractical for complex matrixed displays containing hundreds of dots. In order to reduce the complexity of the circuitry required for providing individual pin connections for each LCD dot, complex clocking schemes must be employed requiring further complex circuitry to develop the required clocking waveforms to energize the LCD matrix to compensate for the absence of the diode isolation effect provided in an LED matrix. Accordingly, there is a need in the art to provide an LCD dot matrix that can essentially operate in a circuit to emulate an LED dot matrix
A 2.times.N LCD matrix control system, for example, is described in U.S. Pat. No. 3,885,861 issued to Farnsworth, et al. on May 27, 1975.
A liquid crystal display device having two layers of LCD cells arranged in a back to back arrangement is disclosed in U.S. Pat. No. 4,139,278 on Feb. 13, 1979 issued to Matsumoto, et al., entitled "Liquid Crystal Display Device." This patent describes (FIGS. 30-40) the pair of liquid crystal cells arranged in superposed relation and disposed between polarizing plates to provide a display that is polarized by the combined effect of controlling the two LCD cells. The pair of LCD cells is sequentially scanned in two orthogonal directions so as to activate any single dot or area for the purpose of converging two lines as in a camera viewfinder. Such an arrangement, however, does not provide for a dot matrix capable of being addressed as an LED dot matrix.